1. Field of the Invention
The present invention relates to a seal ring provided between a ship propulsion shaft and a casing thereof; whereby, the propulsion shaft penetrates the inner space of the casing that is provided on the outboard side of the ship stern tube, the seal ring being used for preventing sea water from entering the inner side of the ship. The present invention also relates to a stern tube sealing device provided with the seal ring.                It is hereby noted that the term “invention” or “present invention” in this specification means the aspect, the mode or the embodiment disclosed by this specification.        
2. Background of the Invention
A seal ring made of rubber or elastomer has been conventionally used in the stern tube sealing device. For instance, Patent Reference 1 (JP2000-238694) discloses a stern tube sealing device as shown in FIG. 7; the device is provided with lip shaped seal rings 04, 05 and 06 made of elastic material, the lip shaped seal rings keeping contact with a liner 03 fitted to a propulsion shaft (a propeller shaft) 02 penetrating a stern tube 01 so that each seal ring is sandwiched by a housing member 07 and an adjacent housing member 07; the first seal ring 04 on the rearmost side of the ship and the second seal ring 05 next to the first seal ring 04 form a first stern tube annular-space 08, and the second seal ring 05 and the third seal ring 06 form a second stern tube annular-space 09; thus, the stern tube sealing device 010 prevents sea water and foreign matters from entering the inside of the ship, as well as, prevents lubricating oil on the inside of the ship from leaking outboard.                The cross-section profile regarding the seal rings 04, 05 and 06 forms the shape such as shown in FIG. 8.[References][Patent References]        
Patent Reference 1: JP2000-238694